Signal seeking receiver for frequency modulated signals



out any action Patented Aug. 16, 1949 SIGNAL SEEKING RECEIVER FOR FRE- QUENCY MODULATED SIGNALS Madison G. Nicholson, Jr.,

to Colonial Radio Corpo Snyder, N. Y., assignor ration, Bufl'alo, N. Y.

Application November 13, 1944, Serial No. 563,117

This invention relates to signal seeking receivers, and more particularly to signal seeking receivers for frequency modulated signals.

A signal seeking receiver, as the term is used herein, means a receiver in which the tuning instrumentality is motor driven, in which the motor is set into operation by the momentary operation of a push button or the like and stopped by the receiver itself when a signal is tuned in, withby the operator.

Such receivers are not diificult to construct for operation on amplitude modulated signals, and will operate very satisfactorily in response to the same. However, if it is attempted to use such receivers for tuning in frequency modulated signals (assuming the receiver is capable of receiving them), a great difficulty immediately becomes manifest. In amplitude modulated signals the carrier frequency remains constant, and since it is the action of the carrier frequency which halts the tuning, a signal seeking receiver will always tune itself very exactly on amplitude modulated signals.

In frequency modulated signals, as is well known, the carrier is constantly changing in frequency when the signal is being modulated, shifting at a rate determined by the modulating frequency and by an amount determined by the intensity of the modulating sound. For 100% modulation in so-called wide swing frequency modulation systems the carrier may deviate or swing as much as 75 kc. on either side of the mean.

It is well known that the necessity of accurate tuning is of extreme importance. Unless the receiver is tuned very exactly to the mean carrier frequency, unbearable distortion and noise may be introduced, and it is generally true that the greater the degree of mistuning the greater the distortion and noise may be. From this it will be appreciated that the problem of providing a signal seeking receiver which will stop exactly on the mean carrier is very complex.

The use of maximum current in a resonant circuit does not lend itself to a solution, because, supposin the resonant circuit to be approaching the mean carrier frequency as its tuning changes, if the carrier happens to be swinging at the time the tuning of the resonant circuit happens to come into the range of the carrier, it is a mere matter of chance at what frequency within the deviation range the circuit will stop; and since the frequency range over which the signal is receivable is much greater than the range over which it is receivable without such 8 Claims. (01. 250-40) noise and distortion, it is apparent that the chances of stopping in the correct range are comparatively small.

In accordance with my invention, I provide circuits so arranged that this effect is entirely eliminated, and the receiver will stop and remain exactly at the mean frequency within the tolerance for which the apparatus is constructed, but if it should by any chance overshoot the correct tuning position and tend to stop in an incorrect position, it will, without any attention on the part of the operator, correct itself and return to the correct position; also, if thermal drift occurs, causing mistuning, the receiver will retune itself, correcting for the drift; and this is done, not by mechanical adjustments of stops in predetermined position, but by the signal itself, according to the signal seeking principle.

From the foregoing, it will be understood that among the objects of my invention are the following:

To provide a signal seeking receiver which will operate on frequency modulated signals.

To provide a receiver for frequency modulated signals which will tune itself exactly within the limits of tolerance of the apparatus to the mean carrier frequency even though the carrier may be of the wide swing type, modulated.

It is a further object of my invention to provide such a. receiver which, should it for any reason accidentally overshoot and mistune the signal, will automatically set itself into operation and correct the overshooting or mistuning.

It is a further object of my invention to provide such a receiver which will automatically correct for the effects of thermal drift as and when it occurs.

It is still a further object of my invention to provide such a receiver in which the additional parts and apparatus required over and above those necessary for a signal seeking receiver for amplitude modulated signals are relatively small in number and in cost.

Still other objects and advantages of my invention will be apparent from the specification.

The features of novelty which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its fundamental principles and as to its particular embodiments, will best be understood by reference to the specification and accompanying drawing, in which Fig. 1 is a circuit diagram of a signal seeking receiver in accordance with my invention, and

Figs. 2, 3, and 4 are curves explaining the operation of my invention.

Referring more particularly to Fig, 1, the signal-s may be received on any suitable form of antenna 10 and then supplied to converter II, in which the frequency is changed by interaction with oscillations generated by oscillator 12. The frequency modulated output of the converter may then be: supplied tointerr'hediat'e' frequency amplifier 13, all 'as' heretofore practiced. The final output of intermediate frequency amplifier is may be passed into the last intermediate frequency coil l3w tuned by condenserjt'efc;

From this point on the construdtiori 'ahdoperation of my receiver departs irom the conventional receiver. The output from. coill3w' fed to a discriminator eom risingtheupper'portion made up of winding ML shunthd byT tuning con-.-

denser l4cL, which may be tuned to a desired frequency lower than the mean frequencyby'the' desired amount.

Coiin'ected ifom 01 1c side (if condenser MCL I may: providediode" i5I-i;havihg cathode l 5L0 and anode IEaL, and cathode IBcL maybe connected through resistance I6 shuntedby by p'ass condenser IBc' to"gr oiihd. Similarly, in the other portion of the discriiiilnatoi ,inductance Mb may be shunted by tuning condenser Mch, and this combination-may be'tuired toanother frequency differing from themean frequency by the same amount as that of Mai. and ML, but-on the high frequency side. 7

One side of condenser ilclz may b'e'connected to thecathode or diode l5hghaving cathode i5ch and anode waft.- Anodelsanmay be connected to ground through resistance l9-= shunted by bypass: condenser I90. '--The oppositeside-oi condenser l iclamay'heconriected to ground through resistance l1 shunted proposes condenser Ho, and the opposite side of condenser 1 46h may be connected to ground through resistance 18 shunted by by-pass condenser I80.

The high-'Voltagesid of each of re'stances l1 and I8 may.- be connectee together through re- 'sistances 2i and 22 respectively. ahdthe high voltage side of: each of resistances. It and I9 may be connected: together through resistances 2e and 23 respectively. The common point of resistances 20 and 23 maybe connected through resistance 24 to the control electrode of thyratron 34, having cathode: 34c, control electrode 349, and anode 34c; and by-pass condenser 26 may be connected iromcontrolelectrode 349 to ground; 1 a

The common point of resistances. 2!, and 22 may be connected thi'ough resistance 25' to the control electrode of anothr'thyratron-35, having cathode 350, control electrode 359, and anode 35a, and also to control electrode too-or control tube 39, which may be a triode. having cathode 300, control electrode 30g, andlanodesllc and control electrode 3llg may'be connected to g'roundthrough by-pass condenser 21. v

The common cathode connection or the thyra trons may be grounded through resistance 28 and also connected through resistance 19 to +B.

Control tube 3|];may operate a lock-in relay, having winding 32w, armature 32, and front contacts 32 The anode 30a. of tube 30 may be connected through resistance 33' to control electrode 349 or thyratron 3'4.

The tuning motorin indicated asat 36, having its shaft coupled to" the tuning instrumentalities controlling the receiver, diagrammatically indicated as a tuning-condenser He in converter II and one [2a in oscillator l2, ganged together with drive connection 368 to motor 36.

The tuning motor may be a reversible induction motor of the capacitor type, having one 5 winding 36]! which may be connected through condenser 31 to an alternating current source,

preferably 115 volt-s, 60 cycles. The action of the capacitor is to cause a substantial phase shift in the current flowing. through field 36H.

If; new, the other winding 36f2 be connected together across the same source, a current will flow through it, but this current will be displaced in phase relation. to that in field 36f! by substantially 90 Thus the action so far obtainable is thesame as that in a two-phase induction motor, with the: direction of rotation being determined' by which of the two possible phase connections used reversal of the connection of one field, of course, causing reversal of the direction of rotation ofthe motor.

If however. Winding 36 2 is center-tapped with the tap being connected to one side of the power source, then the direction of rotation is determined by the'choiceof which end of'the winding is connected to the other side of the power Source. This connection may be established according to the present invention by connecting opposite ends of" winding 3W2 to anodes 34a and 35a of thyratron's 34 and 35 and connecting the mid-point of winding 3th? to a point on the same source having a. voltage of approximately 300 volts. It will now'be understood that the direction of rotation of the motor will be dependent upon which of the thyratrons is ionized.

At this point it maybe noted that, while in ordinary thyratron operation, once it fires it stays fired until the plate circuit'isopened, in the pres ent case, since the plates are fed with alternating current, neither thyratron,. if. fired, will remain firedfor a complete cycle, but each'will fire for one-half cycle if" the control electrode voltage permits it, and will be extinguished in any event at the end of such half. cycle.

The operation of the receiver as a Whole will now be. described. It will benoted that-the discriminator herein has twooutput circuits, one for each thyratron. The characteristic of each circuit'is that of a typical balanced discriminator, each having the opposite polarity of the other. This is shown inv Figs. 3 and 4, in which Fig. 3 represents the voltage at point E as applied to control electrode 34g of thyratron 34. While the output of each circuit is zero for exact tuning, for detuning in one direction the output of onev circuit is positive. while that of. theother is negative and for detuning in the. opposite direction the polarities of the outputs are likewise reversed.

The response oi each circuit isabout one-half of maximum when the signal is at center frequency. One. diode rectifier is used to rectify the output of each circuit. Instead of a single diode load resistor for eachdiode, however, two load resistors of equal value are: used, one from cathode to ground and the other from anode to ground. For diode I5L these load resistors are Iii-and H; for diode l5h they are I9a'n'd Hi.

In this way thereare obtained from each diode two equal outputs, one being positive with respect to ground; the other being negative with respect to ground. This is clearly shown in Fig. 2, in which curve A represents thevolta'ge from cathode 15130 to ground and that'oi curv B the volt- 7 age from the high side of resistor l-l to ground.

Similarly, curves and D represent respectively the voltage from the high side of resistor I8 to ground and the voltage from anode lah to ground. By adding outputs A and D by means of resistors 20 and 23 at point E, the curve E, Fig. 3, is obtained. Likewise by adding outputs B and C through resistors 2| and 22, the output at point F is obtained as shown by curve F, Fig. 4. The voltages at points E and F are audio frequency voltages corresponding to the modulation, and either may be supplied to the audio amplifier and loud speaker (not shown) or since these voltages ar of opposite phase, they may both be used to supply a push-pull audio amplifier.

The cathodes of the thyratrons are biased just enough so that the output of the discriminator will cause one thyratron to operate if the receiver is detuned by more than the desired tolerance. Since outputs E and F are opposite in polarity, only one of the two can be positive at any time; thus only one of the thyratrons will operate when the receiver is detuned.

By connecting the motor electrically and mechanically so that detuning will cause the motor to drive the tuning mechanism in a direction toward the point of proper tuning, a signal may then be tuned in by the action of the discriminator, the thyratrons, and the tuning motor. It will be seen, however, that this action will not occur unless the receiver is tuned within the zones covered by curves E and F of Figs. 3 and 4.

It is, therefore, necessary that an additional action b provided to carry the receiver into the zone where this action may occur. This is accomplished by a means which causes one of the thyratrons to operate until the tuning reaches curves E or F, which may be termed the fine tuning zone. For this purpose one of the thyratrons is caused to operate prior to receivin a signal, and this is accomplished by triode 3!], push button switch 3!, and lock-in relay 32.

Assuming that the receiver is tuned to a frequenoy far removed from that of an incoming signal, if push button 3| is momentarily depressed, plate voltage is applied to triode 30, plate current flows, and relay armature 32a swings to front contact position and locks in. Opening of push button 35 will not disturb this condition. The characteristics of triode 30 are so chosen that the relay will remain locked in as long as no negative bias is applied to the control electrode of the triode, applying a positive bias to the control electrode 349 of thyratron 34 from +B, through winding 3210 and resistance 33, and the tuning motor will operate to vary the tuning of the circuit.

As soon as a signal is partially tuned in; i. e., the tuning reaches the fine tuning zone, a negative bias is applied to the triode, said bias obtained from the discriminator outputs through resistance 25, causin the plate current of triode 39 to drop sufiiciently for the relay 32 to open. This removes the +B voltage which previously had been applied through resistor 33 to control electrode 349 of thyratron 34 to keep the same in operation, and control of these thyratrons is, therefore, now effected only by the output of the discriminator.

Suppose, for instance, that relay 32 opens at the frequency designated by arrow I in curves of Figs. 2, 3, and 4. It will be observed that the output voltage applied to thyratron 34 is positive, whereas that applied to thyratron 35 is negative. Therefore, thyratron 34 will continue to fire until the tuning reaches a point designated by arrow 2, at which time thyratron 34 can no longer continue to fire because of insufficient positive voltage on its control electrode, no curent will flow in the tuning motor field, and the motor will accordingly, come to rest.

Suppose that its inertia is such as to carry it to the frequency designated by arrow 3. Before this point is reached, the voltage impressed on thyratron 35 has changed from negative to positive, as will be observed in Fig. 4, and that on thyratron 34 has changed from positive to negative, as indicated in Fig. 3. As soon as thyratron 35 begins to fire, if the motor has not stopped rotating, the field currents passed by thyratron 35 will exert a braking action on it, and will cause it to stop and finally to rotate in the opposite direction until it passes into the tolerance zone.

It will be apparent that, should the inertia of the motor be sufiiciently great, a hunting action might occur, in which the motor would continue to rotate, first in one direction and then the other, swinging the tuned frequency continuously back and forth about the center frequency. To prevent this, it is desirable that the inertia of the parts be kept small so that excessive coasting after the cutoil of motor current occurs will not take place.

It will also be clear that once the system has tuned itself to the center frequency of the discriminator as described, excessive percentages of modulation will not cause the receiver to change its tuning to follow the carrier as it swings about the mean frequency, because any currents flowing through the motor, if integrated. over a period long enough to overcome the motor inertia and cause rotation, will equal or closely approximate zero.

In this connection it may be noted that the RC combinations, resistance 24 and condenser 26, and resistance 25 and condenser 27, are preferably so chosen that modulation frequencies in the output of the discriminator are by-passed to ground and do not affect the tuning.

An interesting feature of my invention is that the circuit is substantially immune to the harmful effect of thermal drift. As drift occurs, its result in the output of the discriminator is the same as mistuning. When it exceeds the tolerance limits of the circuits, one or the other of the thyratrons will begin to fire, and the motor will operate to the extent necessary to retune the receiver until the mean carrier frequency again has the desired value.

Consequently, it will be seen that the action of the circuit is that stated to be desired; i. e., by pushing button 3! the tuning variation is set into operation and will continue until the receiver tunes in a frequency modulated signal of predetermined strength, at which time the fine tuning comes into play and the receiver will tune itself to the mean carrier frequency.

In this application I have explained the principles of my invention and the best mode in which I have contemplated applying those principles, so as to distinguish my invention from other inventions; and I have particularly pointed out and distinctly claimed the part, improvement, or combination which I claim as my invention or discovery.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from the spirit and eager? scope thereof, as willbeclear to those skilled in the,art TI, 1 2 .L Iclaim: 1. Radio receiving apparatus comprising, in combinatiommeansfor receiving frequency modulated signals, including a variable tuning instrumentality for selecting the signal to be received, electric power operated means'for drivingsaid tuning instrumentality, manual means for starting said power operated means, a frequency diseliminator-rectifier network supplied by received signals, and-means operated by the received 's'i'g' nal and responsive to the degree of detuning' of said received signal'within the operating frequency range of the frequency discriminator-rec tifier'network for correcting said detuningfsaid last mentioned 1 means comprising a "pair of thyratrons for causing operation of said power operated means in opposite directions, and means for applying a firing control voltage of opposite polarity derived from the output of said frequency discriminator-rectiiier network" to each of said'thyratrons respectively. "1:2; Radio receiving apparatus comprising, in combination, means for receiving frequency modulated signals,- including a variable tuning instrumentality for selecting the signal to be re ceived, electric power operated means for driving said tuning instrumentality, manual means for starting said power operated means, a frequency discriminator-rectifiernetwork supplied by received signals, means operated by the'received signal. andresponsive to the degree of detuning of isaid received signal within the operating fre quen'cy range of the frequency discriminator rectifier network for correcting said detuning,'"a pair of'thyratrons for causing operation of said power'operated means in opposite directions, and means for applying a firingcontrol voltage of opposite polarity from the output of said frequency discriminator rectifier network to each of said thyratrons respectively.

:3; Radio receiving apparatus comprising, in combination, means for receiving frequency modulated'signals, including a variable tuning instr'u' mentality for selecting the signal to be received, an electric motorfor driving said tuning instrumentality in opposite directions, manual means for starting said power operated means, a frequency discriminator-rectifier network supplied" by received signals, means operated by the received signal and responsive to the degree of detuning of said received signal Within the operat ing' frequency range of the frequency discrimi hater-rectifier network for correcting said "detuning, said means being responsive botnin direction' and'magnitude to-tne degree of detuning of-the carrier within the operating frequency range of the frequency discriminator rectifier network, and means for preventing response ofsaid power operated means to carrier deviations tuning instrumentality, manual means forstart ation'of said power mean in opposite directions, saidpower means comprising a pair of windings 'for'operating' said power means in opposite directions, each of said pairof windings being sup plied by one of said thyratrons.

-"- 55Radio receivingapparatus comprising, in combination, means for receiving frequency mod; ulated signals, including a variable tuning instru; mentality'for selecting the signal to be 'recfeived, electric 'pow'eroperated means for driving said tuning instrumentality, manual means for starting-said power'operated means, a frequencyfdislcriminator-rectifier network supplied by received signals, a first control meansfor providingcon: tinued operation of said'variable tuninginstr'umentality'after it has been started and until the variable" tuninginstrumentality reaches the operating frequency range of the frequency dis; criminator-rectifier network, and a second con trolling instrumentality operable in thepperatk 1 ing'frequency range of the frequency discrimiing-said power operated means, a frequency dis-ft criminator-rectifier network supplied byreceived signals, a first control meansfor providing continued operation of saidvariable tuning instrumentality after it has been started and until the variable tuning instrumentality reaches the'op l crating-frequency range of the frequency discriminator-rectifier network, said'means' comprising a lock-in'relay energized by said manual means; and asecond controlling instrumentality" operable'in the operating'frequency rangejo'f the frequency discriminator rectifier network for rendering the first control instrumentality inoperable and for subsequently 'controlling'tuningof.

the signal.

7, Radio receiving apparatus comprising, in combination, means for receiving frequency modulated signals, including a variable tuning instrumentality for selecting the signals to be received,

electric power operated means'for drivingsaid tuning instrumentality, manualmeans forstarting saidrpower operated means, a frequency dis-- crim inator-rectifier network supplied by received signals, a first control means comprising a lockin relay: for providing continued operation of saidv variable tuning instrumentality after it has been startedand until the variable tuning instrumeni-v tality reaches the operating frequency, range of the frequency discriminator-rectifier network,

and a second I controlling instrumentality come j prising a frequency discriminator rectifier net:

work and a pair ofthermionic discharge devices.

for rendering the first controlinstrumentality inoperative and for controlling tuninsof the sig-,

nal within the operating frequency range, ofthe frequency discriminator-rectifier network.

ing said power operated means, and means responsiveto received signal for stopping variation of said tuning instrumentality when a frequency modulated signal of predeterminedstrength is tunedin; saidlast mentioned means comprising a .theoutput of said discriminator for causing'oper mentality for selecting the signals to be received.

electric power operated means for dnivingjsaid. tuning instrumentality, manual means for start if ing said power operated meansfa frequency dis}-,j criminator-rectifier network supplied byreceivedl, signals, apair of thyratrons for controllingthejf direction of operation of said tuning instrumentality, a discriminator for applying bias voltage of opposite polarity to said thyratrons respectively to control the tuning of the signal over the operating frequency range of the frequency discriminator-rectifier network, a lock-in relay for applying firing voltage to one of said thyratrons, and a thermionic discharge tube for controlling said lock-in relay, and means for applying a. bias voltage from said discriminator to said control tube to unlock said relay when the variable tuning instrumentality reaches the operating frequency range of the frequency discriminator-rectifier network.

MADISON G. NICHOLSON, Jn.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,700,282 Burns et a1 Jan. 29, 1929 Number Number 10 Name Date Bruckel et a1 Nov. 4, 1930 Somers Aug. 20, 1935 Lowell Oct. 6, 1936 Williams, Jr. Apr. 5, 1938 Conover Mar. 14, 1939 Van Loon Mar. 28, 1939 Bernard Sept. 19, 1939 Case Oct. 3, 1939 Hansell July 29, 1941 Martin et a1 July 29, 1941 Duinker Mar. 24, 1942 Reid Apr. 14, 1942 Alexandersson Apr. 14, 1942 Thompson May 12, 1942 Alexandersson et al. June 8, 1943 Peterson May 30, 1944 Goldstine Sept. 19, 1944 Dietrich Feb. 13, 1945 FOREIGN PATENTS Country Date Great Britain Feb. 25, 1929 

